The present invention relates to a system for controlling an electromagnetic clutch for vehicles, and more particularly to a system for an electromagnetic powder clutch of the type in which clutch current is controlled in dependency on the vehicle speed detected by a speed detecting means.
An electromagnetic powder clutch as a type of electromagnetic clutch comprises an annular drive member secured to the crankshaft of an engine, a driven member installed on an input shaft of a transmission spaced from and leaving a small gap from the drive member, and a magnetizing coil provided in one of the members. A shift lever changes gears in the transmission. The shift lever is provided with a switch for the magnetizing coil, which is actuated by operating the shift lever. As the accelerator pedal of the vehicle is depressed, the current applied to the coil increases. The magnetic powder is aggregated in the gap between the drive member and the driven member, bring about a connection between the drive member and the driven member. The clutch current passing through the magnetizing coil is progressively increased depending on how far the accelerator pedal is depressed, while the clutch slips between the drive member and the driven member and gradually engages when the clutch current reaches a rated current. Thus, the vehicle may be started by depressing the accelerator pedal without operating a pedal switch.
In order to control driving of the vehicle, it is necessary to detect the speed of the vehicle and to control the clutch current according to the speed. In a system to meet such a requirement, a speed sensor is provided to generate a signal when the vehicle speed exceeds a predetermined value. The system is designed such that the clutch slips so as to provide semi engagement of the clutch during low speed below the predetermined speed and the clutch engages perfectly when the vehicle speed reaches the predetermined speed. When the vehicle speed decreases to a predetermined low value, the clutch is disengaged for preventing stopping of the engine. Generally, the speed sensor is provided in a speedometer. In systems employing such a speed sensor, there is the disadvantage that, if the speedometer cable is broken, the speed sensor does not work. Therefore, if the speed sensor does not work in a deceleration condition, the circuit of the clutch coil is interrupted so that the clutch disengages. As a result, the braking action of the engine does not occur.